The present invention relates to a microstructure and its manufacturing method.
Filter membranes including organic membranes and inorganic membranes are commercialized in the field of microfiltration, and such organic membranes are actually widely used. In most of the organic membranes, pores are not separated from each other and the pore size distribution is relatively broad. Therefore, researches for further improving the accuracy in the separation of a target substance which is the most important filter function have been made in various fields.
In order to solve such problems, a so-called track etching technique is known in which an organic film made of a polymer is irradiated with high energy particles generated in a nuclear reactor, and tracks of the particles through the organic film are etched to form micropores (see T. D. Brock, Membrane Filtration, Sci. Tech, Inc., Madison (1983)). According to the track etching technique, discrete micropores with a narrow pore size distribution are formed orthogonally to the organic film, but this technique suffered from the problem that the pore density, that is, pore opening area ratio could not be increased to prevent overlapping pores from being generated due to incidence of particles on the film in an overlapping manner when forming tracks.
On the other hand, a porous alumina membrane filter making use of an anodized film of aluminum, such as the one described in Hideki Masuda, “New Technology of Porous Membranes Using Anodization” (ALTOPIA, July 1995) is known for the inorganic membrane. Aluminum is anodized in an acidic electrolytic solution to dispose discrete micropores having a narrow pore size distribution to achieve a high pore opening area ratio, so a membrane filter with a high filtration rate per unit time can be produced at low cost.
Specific examples of the application of such organic and inorganic membranes include a protein adsorbent, water purification, air washing, a structure for deodorization/denitration/exhaust gas equipment, ozone removal, removal of various viruses, a clean room material, gas separation, a red cell deformation test, a radioactive assay of chemotaxis using a culture chamber and a scanning electron microscope, various treatments in tests using, for example, exfoliative cytology, fluorescent X-rays and et al., alkaline elution, monitoring of asbestos, detection of a parasite, IR analysis of particles in the air, and analysis of algae in a river or sea. Such membranes may also be used in fractionating slurry particles, pigments, dyes, and magnetic substances used in chemical mechanical polishing (CMP).
However, improvements to the porous alumina membrane filter have been desired because of the fact that the structure of the porous alumina membrane filter includes an anodized film of aluminum, and when an aqueous solution is separated over a long time, hydration of the anodized film occurs, which may result in a reduced pore diameter and insufficient stability with time of the filtration rate.